1. Field of the Invention
The present invention relates to a fully automatic passive rotational alignment system for the splicing of polarization-maintaining single mode fiber.
2. Description of the Prior Art
Polarization-maintaining single mode fibers present special difficulty in splicing or coupling. Ordinary single mode fibers must be aligned with each other laterally and longitudinally to within about 1 micron, and in angle to within a fraction of a degree. Polarization-maintaining fibers must also be aligned azimuthally; that is, they must be rotated relative to each other about their common axis until the fast and slow axes in their respective cores are also aligned. This is because it is essential for the successful application of these fibers that the transmitted light remain in the preferred polarized mode--either fast or slow--in crossing the splice. If the mode alignment is off by more than about 1 degree very serious losses take place. Not only does the projected power become divided between the two orthogonal modes of the receiving fiber, but the coupling into the originally-excited mode is very poor. An effective means of aligning such fibers is thus clearly desirable.
In U.S. Pat. No. 4,612,028, there is disclosed a polarization-preserving single mode fiber coupler made without mutually aligning the polarization axes of the fibers by twisting the fibers together over a selected length and fusing them. A critical requirement of this coupling method is that the initial misalignment be not close to 90 degrees.
As taught in U.S. Pat. Nos. 5,156,663 and 4,911,524, the principal manner of aligning polarization maintaining single mode fibers has heretofore been to rotate a first fiber relative to a second fiber while exciting the first fiber and monitoring the output from the second. That is, the first "transmitting" fiber must be aligned with a polarized light source for injection of light aligned with the preferred axis. Likewise, the output end of the "receiving" fiber must have its preferred axis aligned with a polarizing filter and detector. Thereafter, the ends of the fibers to be spliced or coupled are brought together in a suitable stage or housing, for instance on a fusion splicer. After the ends have been aligned laterally with each other in x, y, and z dimensions, to maximize the coupling of power across the gap, one fiber is rotated slowly relative to the other while the power received at the photodetector is monitored. Eventually an orientation is found at which the coupling of power into the preferred axis is optimum. The fusion or mechanical splice is then completed, by fixing the oriented ends together permanently.
U.S. Pat. No. 5,244,977 to Anjan, et al. discloses a fiber optic polarization apparatus for use in the fabrication of fused optical couplers. U.S. Pat. No. 5,013,345 to Itoh, et al. discloses a method for fusion splicing of polarization maintaining optical fibers, while U.S. Pat. No. 5,149,350 to Itoh, et al. discloses an apparatus for fusion splicing of optical fibers. In each of these systems, light is injected into a free end of one fiber and detected at a free end of the second fiber. The fibers are aligned by monitoring the light transmitted through the joining surfaces as a function of angle.
U.S. Pat. No. 4,669,814 to Dyott discloses an optical fiber comprising a core and cladding having different refractive indices and forming a single-mode guiding region, where the core has a noncircular cross-section defining two refractive indices. Like the Anjan, et al. and Itoh, et al. systems, the Dyott system discloses a fiber in which light is injected along its length. The injection of light in the Dyott system is accomplished by a beam splitter.
Coupling of fibers using the methods described hereinabove require light to be injected along the length of the joining fibers. Rotational alignment is, alternatively, accomplished by the following methods: (1) coupled power monitoring, which is difficult and time consuming, and requires expensive input and output source and detector alignments; (2) axial imaging, which requires the fiber to have obvious and distinctive features; and (3) lateral imaging, in which the fiber must have obvious internal features amenable to a precisely-alignable image. With the first (power injection/detection) fiber alignment method, the set-up required to power and monitor the fibers is difficult and time-consuming to establish. Highly-skilled personnel are required; and the splicing procedure is itself time-consuming. If more than one pair of fibers is to be spliced, the process time and procedural difficulty increase dramatically. Methods (2) and (3) depend on imaging distinctive internal physical features of the fibers. If alignment is to be automatic, the system therefore requires sophisticated, expensive, and delicate image processing technology. If it is to be manual, the ability of a user to visually match two low-contrast images is oftentimes not accurate enough to yield rotational alignments of the required precision of 1 degree or better. Furthermore, few pm fibers exhibit images with distinguishable features, either in the axial or lateral views. Thus methods (2) and (3) are both difficult to implement and limited in applicability to a small proportion of the available pm fibers.
There remains a need in the art for an improved method for coupling polarization-maintaining single mode fibers.